Interference rejection circuit



June 18, 1940. E. l. ANDERSON Er AL INTERFERENCE REJECTION CIRCUIT FiledJan. 4, 1939 INVENToRs EARL l ANDERSON A/vo 77QRD MU/VTJOV /wvf/L/ATTORNEY.

Patented June 18, 1940 INTERFERENCE REJECTION CIRCUIT Earl I. Anderson,Bayside, and Garrard Mountjoy,

Manhasset, N. Y., assignors to Radio Corporation of America, acorporation of Delaware Application January 4, 1939, Serial No. 249,200

6 Claims.

t Our present invention relates to signal selector circuits, and moreparticularly to tuned selector circuits having substantially inniteattenuation for undesired adjacent channel signals. In U. S. Patent No.2,137,475, granted Nov. 22, 1938, G. Mountjoy has disclosed and claimedan intermediate frequency (I. F.) transmission network, for asuperheterodyne receiver, which includes two cascaded I. F. resonantcircuits. The rst of the circuits is designed to reject substantiallycompletely one adjacent undesired signal channel, and the followingcircuit is constructed to reject the adjacent channel on the oppositeside of resonance. As disclosed in said patent there exists somemechanical complexity in the manipulating elements; as a result acertain degree of operating skill is required to effect the ejection ofadjacent channel signals.

One of the main objects of our present invention is to provide anautomatically operating network o-f the aforesaid infinite attenuatortype; the system employed eliminating the need for especial skill on thepart of the receiver operator and taking advantage of the naturaltendency to 25 attempt to retune the receiver when interference occurs.

l Another important object of our invention is to replace the couplingcapacities of the innite attenuator network by tubes acting aselectronic 3U capacities; a frequency discriminator responsive tovdetuning of the receiver, with respect to a desired carrier frequency,being used to control the magnitude of either of the electroniccapacities to an extent such as to provide practically completerejection of an undesired adjacent channel signal.

Still other objects of this invention are to improve generally theselectivity and transmission efficiency of coupled signal selectorcircuits, and

40 mor'e especially to provide selector circuits of substantiallycomplete rejectivity against adjacent channelinterference and whichselector circuits are not only automatic and simple in operation,` butare also readily assembled in radio receivers.

'The novel features which we believe to be characteristic of ourinvention are set forth in particularity in the appended claims; theinvention itself, however, as to both its organization '50 and method ofoperation will best be understood byreference to the followingdescription taken in connection with the drawing in which we haveindicated diagrammatically a circuit organization whereby our inventionmay be carried into effect.-

In the drawing:

Fig. l diagrammatically shows a receiving system embodying theinvention,

Fig. 2 graphically illustrates the functioning of the invention.

Referring now to the accompanying drawing, there is shown in Fig. 1 asuperheterodyne rereceiver of a type generally known. The receiver maycomprise a signal collector A; and the latter may be the usual groundedantenna circuit, a radio frequency distribution line, or even the usualsignal collector employed on an automobile receiver. The numeral Idenotes a networkcomprising a radio frequency amplifier whose output isfed to a rst detector. The input circuit 2 includes the usual type oftuning condenser 3. It will be understood that the first detector willalso include a tunable input circuit, and that the latter will beprovided with a variable tuning condenser'. There is impressed upon thefirst detector locally produced oscillations from the local oscillatorl. The oscillator tank circuit 8 includes a variable tuning condenser 9,and the dotted line I0 designates the usual mechanical uni-controldevice which functions to vary the positions of the rotors of the threevariable condensers employed in the tunable circuits of network I andthe tank circuit 8.

Those skilled in the art are fully acquainted with the manner ofconstructing the aforesaid portions of a superheterodyne receiver; theyare, also, aware of the fact that the three tunable circuits are sorelated that the signal energy appearing in the output circuit I I ofthe first detector has a frequency value equal to that of the operatingintermediate frequency (I. F.). The I. F. value may be chosen from arange of '75 to- 460 k. c., and the frequency of the oscillator tankcircuit 8 must constantly differ from the frequency of the tunablesignal circuits by a value 40 equal to the I. F. It is to be clearlyunderstood that instead of using separate tubes for the first detectorand oscillator, a tube of the pentagrid converter type (2A7) may be usedto provide a combined local oscillator-first detector network. 4'5 TheI. F. output circuit II is followed by an I. F. amplifier' I2, which maybe of any well known type. A second I. F. amplifier I2 follows amplierI2, and the subsequent second detector network comprises diodes I3 andI3 arranged in an 50 electrical relation to be described at a laterlpoint.

Between the I. F. output circuit II and the input electrodes ofamplifier I2 there is connected the tuned circuit I4, and the latter isresonated to the operating I. F. Input circuit II comprises 55r circuitsI I coil IE and the shunt tuning condenser I5; tuned cir'cuit I4comprises coil I'I and the link coupling coil I8, coils I'I and Ilhaving connected in shunt therewith the resonating condenser I9. Thecapacity C connects the high alternating potential sides of the circuitsI I and I4, and provides a capacity coupling path between the cascadedtuned circuits. Inductive coupling is provided between circuits I! andIl by the coupling between coils I6 and I8, and this inductive couplingis denoted by the symbol M. The circuit details of amplifier I2 are notshown, but it will be understood that the cathode circuit thereof, asshown in connection with amplifier I2', includes the usual signal gridbiasing network. The I. F. output circuit /.I is connected to the plateof the amplier I2.

The coupling capacity C is denoted in dotted lines, because it isprovided by the capacity between the grid and cathode of tube 3B. Thecontrol grid of tube 39 is connected to the high alternating potentialside of circuit I I by a direct current blocking condenser 3l, whereasthe cath- 0de of tube 30 is connected to the high potential side ofcircuit Ill. The plate ol` tube 3] is connected to a source of positivepotential through a resistor 32. The magnitude of capacity C iscontrolled by varying the bias of the grid of tube 3G.

The inductive coupling M is so poled that the voltage induced through Mis opposite in sign to the voltage induced through the capacity couplingC. Furthermore, these voltages are made to cancel at an adjacent channelfrequency which is spaced from the I. F. value by a predeterminedfrequency magnitude. In Fig. 2 there is shown the type of selectivitycharacteristic secured with the circuit disposed between the network Iand amplifier I2. It will be understood that the curve in Fig. 2 to oneside of the vertical, dotted center line represents the rejectioncharacteristic of the coupling network between I and I2. The resistor R,connected across the link coupling coil I8, functions to correct for allfactors in the circuit. In other words, the function of the resistor Ris to provide accurate cancellation of the opposed voltages due to C andM; correct values of M, C and R provide infinite rejection of theundesired adjacent channel frequency.

The resistor 33, connected between resistor R and ground, is shunted byby-pass condenser 3/4 and functions as the grid biasing network for thegrid of tube 3B. An examination of Fig. 2, and particularly one-half thecurve on either side of the central vertical line, shows the infiniterejection characteristic secured. This type of rejection characteristicis seciu'ed when the capacity C is adjusted to reject an adjacentchannel frequency which is 10 k. c. off resonance. It may be pointed outthat while the capacity coupling C increases with frequency, impressedvoltages transmitted through the coupling M are subjected to adecreasing coupling with frequency increase.

Accordingly, while no coupling exists between and I4 at the channel 10k. c. off resonance, coupling does exist at the I. F. The networkbetween amplifier I2 and amplifier I2', on the other hand, isconstructed to produce a rejection characteristic as shown in theopposite half of the curve in Fig. 2. In other words, the couplingcircuit between networks I and I2 has the characteristic shown to oneside of the vertical dotted line in Fig. 2, while the coupling circuitbetween amplifiers I2 and I2 has the characteristic curve shown in theopposite half of the curve in Fig. 2. It will be seen that the couplingnetwork between amplifiers I2 and I2 rejects the adjacent channelfrequency on the opposite side of the I. F.

The coupling network between amplifier I2 and amplifier I2 comprises theoutput circuit ZI and the input circuit 22 of amplifier I2. The circuitZI is tuned to the I. F., and the coil 23 thereof magnetically coupled,as at M1, to the link coupling coil 2li of the I. F. input circuit 22.The capacity C1, shown in dotted lines, provides the coupling capacitybetween the high alternating potential points of circuits 2l and 22, andis provided by the capacity existing between the grid and cathode oftube 4D. It will be noted that the construction of the selector networkbetween arnpliers I2 and I2' is substantially similar to that precedingamplifier I2.

The control grid of tube 4D is connected to the high potential end ofcoil 23 by a direct current blocking condenser 3|', and the plate of thetube Fill is connected to the positive potential source of tube 3uthrough a resistor 32'. The cathode of tube Ill is connected to the highpotential side of circuit 22. The resistor R1 is connected in shunt withcoupling coil 2d, and functions in the same manner as described inconnection with resistor of tube 4D. Circuits ZI and 22 are eachresonated to the operating I. F. value. I2' includes in its groundedcathode circuit a signal grid biasing network 50, and its control gridis connected by the direct current blocking condenser 5I to the highpotential side of input circuit 22. The plate circuit of tube I2includes an output circuit 52 which is resonated to the operating I. F.value.

The second detector network is of the type which is disclosed andclaimed by S. W. Seeley in U. S. P. 2,120,103 granted June 21, 1938.Since this type of frequency discriminating network is well known, it isnot believed necessary to describe the circuit in great detail. Thecommon input circuit 53 has one end thereof connected to the anode 5ftof diode I3, while the opposite end of the input circuit is connected tothe anode 5E of diode I3. The midpoint of the coil of circuit 53 isconnected through the large condenser 56 to the high potential side ofcircuit 52. The primary coil 51 is magnetically coupled to the secondarycoil 58, and the cathodes oi` diodes I3 and I3 are connected by seriesresistors 59 and 6I). The cathode end of resistor G0 is at groundpotential. The resistors 59 and 6U are shunted by a by-pass condenserEI, and the junction of resistors 59 and 5I] is connected throughcondenser E2 to one side of condenser 6I.

The potential drop across resistor 59-60 will depend in magnitude andpolarity upon the amount and sense respectively of the frequency shiftof the intermediate frequency energy from the operating I. F. value ofcircuits 52 and 53. In the type of discriminator network shown theprimary and secondary circuits 52 and 53 are so connected that twovector sum potentials of the primary and secondary voltages may berealized. At I. F. resonance the intermediate frequency voltagesimpressed on the diode rectifiers.

i3 and I3' are equal in magnitude, but opposite in phase; this relationexists by virtue of the connection shown, and is clearly described inthe aforesaid Seeley patent. Since the rectiers are in seriesopposition, the direct current potential developed across resistors 59and 6U is zero at I. F. resonance. If, now, the intermediate fre- The I.F. amplifier quency energy developed at circuitV 52 departs from the I.F. resonance, a phase shift of 90 degrees occurs. The intermediatefrequency voltages induced, in that case,in the two halves of coil 58are still equal in magnitude and opposite in phase with respect to themidpoint of coil 38. However, the voltage drop across the primarycircuit 52 is noW added vectorially to the induced voltages. Thus, thepotential at one side of the secondary 58 will be the sum of the voltageinduced across one half the'coil 58 and the voltage developed acrosscircuit 52.

The potential of the other side of the coil 58 will` be the differencebetween the voltage drop in theprimary circuit 52 and the voltageinduced in the opposite half of secondary coil 58. In the last case,then, the input voltage to one of the diode rectifiers is muchgreaterthan that to the second one. Hence, the voltage drop across one of theoutput resistors will be greater than that across the other one. Forexample, when the voltage drop across resistor 59 exceeds that acrossresistor 50, then 'the cathode end of resistor 59 will be positive withrespect to the grounded end of resistor 60. When the intermediatefrequency energy impressed on primary circuit 52 is off resonance in thepositive direction, then the cathode end of resistor 55 becomes negativewith respect to ground. The sense of detuning of the receiving systemthus determines the polarity of the cathode end ofv resistor 59; and theamount of detuning determines the magnitude of the bias at the cathodeend of resistor 59.

The gain of veach of tubes 30 and it is controlled in response to thepotential variation at the cathode end of resistor 55. 'Ihe grids oftubes 35 and 40, are, therefore, connected through .s

lter resistors lg, l l as well as through the filter network 12, to thecathode end of resistor 59. It Will now be observed that the signal gridcircuits of tubes 30 and 4B are completed to ground through the seriesresistors 59 and G5. The audio modulation on the I. F. carrier isderived from the junction of resistors 59 and 65; AVC. bias is alsoderived from this junction point. Each of the signal grids of amplifiersi2 and l2 is connected by the AVC. lead,'through proper filterresistors, to the junction of resistors 59 and 65. It will be noted thata radio frequency choke coil is connected between the midpoint of coil58 and the junction of resistors 59 and 50.

To explain the operation of the receiving system shown in Fig. 1, let itbe assumed that the selectivity curve shown in Fig. 2 is that of thereceiving system with an incoming signal properly tuned to; in that caseno direct current voltage is developed across resistors 59-60. Thedesired signal carrier may then be represented as being located at thepoint X which is at the central dotted line of the curve. Assumingfurther nowthat an' interfering signal occurs at a point B, the naturaltendency of the operator would be to adjust the tuning device IU so asto retune the receiver to attempt to reduce the magnitude of theinterfering signal. If the desired carrier were shifted to a pointcorresponding to point Y, the undesired signal would be shifted acorresponding amount to point D. However, upon adjusting the tuningmechanism Hl so as to shift the desiredsignal carrier frequency to pointY, there would be developed sufcient discriminator voltage across 5.9-60to move the rejection notch E to the position F located between thevertical dotted lines B and D. The effect of this shift of the rejectionnotch E to the position F would be entirely to remove the interferingcarrier.

In the same' Way if an interfering signal should appear at point H, itcould be removed by'adjusting the tuning lmechanisn'i l5) until thedesired carrier were shifted to the point I. In other words,'uponadjusting the tuning mechanism I0 in a sense to tune out the interferingcarrier H, the rejection notch G is shifted towards the position of theinterfering carrier I-I, as was described in connection with therejection notch E. While shifting of either of the rejection notches,.say E, towards the center line X results in a simultaneous shift of therejection notch G to the point J, the proper operation of the systemwill not-be affected. The essential advantage of the present arrangementis that the discriminator functions automatically vto adjust themagnitude of either of the capacities C or C1 when the tuning device lilis adjusted to detune the receiver on either side of a desired carrierfrequency.

The effect of the detuning is to develop discriminator voltage; thelatter is utilized for automatically adjusting the appropriate one ofthey rejection networks in a sense to eliminate the interfering carrier.Minimum interference occurs when rejection notch E and the interferingcarrier coincide in frequency. This would occur when the desired carrieris tuned to Y. The interfering carrier has been moved to point D. Bydetuning the desired signal to Y a positive voltage isl developed by thediscriminator circuit; this increases the electronic capacities of tubes30 and 40. In turn, this moves notch E to D which eliminates theinterfering carrier. At the same time notch G would have moved to J butthis effect in this case is unimportant. If, however, thev interferingcarrier had appeared at H the desired signal would have been tuned to I;and the -discriminator output would have been negative and notch G wouldhave moved in to take out the interference. As a practical matter thereceiver operator, upon tuning to a desired carrier frequency andhearing an interfering carrier, will adjust the tuning device lll untilthe interfering carrier response is a minimum. This means that he hasdetuned the receiver suliciently to one side of the desired carrierfrequency to develop enough discriminator voltage thereby to shift arejection notch to the position occupied by the interfering carrierfrequency.

While We have indicated and described a system for carrying ourinvention into effect, it will be apparent to one skilled in the artthat our invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of our invention, as set forth inthe appendedclaims.

What we claim is:

1. In combination with at least two resonant signal circuits each tunedto a common operating frequency, at least two reactive coupling pathsbetween said circuits which are poled in phase opposition, said circuitsbeing characterized by the ability substantially completely to reject afrequency spaced from said operating frequency, and means, responsive toa frequency shift of the signal cnergy'frorn said operating frequency toa frequency intermediate said spaced frequency and operating frequency,for automatically shifting said spaced frequency towards saidintermediate frequency.

2. In combination, in a signal transmission network, a rst pair ofresonant circuits, at least two reactive paths coupling said pair ofcircuits in phase opposition relation so that complete rejection iseffected of a frequency spaced from the common operating frequency ofsaid pair of circuits by a predetermined frequency value, a second pairof resonant circuits in cascade with said first pair, at least tworeactive paths coupling said second pair of circuits in phase oppositionto reject the frequency on the other side of said operating frequencywhich is spaced from the latter by said frequency value, and means,responsive to a frequency shift of the signal energy from said operatingfrequency value, for automatically shifting one of said spacedfrequencies towards said operating frequencies.

3, In a superlieterodyne receiver of the type comprising at least twocascaded intermediate frequency energy transmission networks; the methodcomprising transmitting th-e energy through the first of said networkswith substantially complete rejection of energy of a frequency spaced bya predetermined frequency value from said intermediate frequency,transmitting the energy through the second of said transmission networkswith substantially complete rejection of energy of a frequency valuespaced by an amount equal to said rst frequency separation but on theother side of said intermediate frequency, and automatically decreasingthe frequency separation between the intermediate frequency and arejection frequency upon a frequency departure of the intermediatefrequency energy from said intermediate frequency.

4. In a superheterodync receiver of the type provided with anintermediate frequency transmission network; the method which includestransmitting intermediate frequency energy through said network withsubstantially complete rejection of signals of a frequency spaced by apredetermined amount from the interme diate frequency value, andautomatically decreasing the frequency spacing magnitude between therejection frequency and the intermediate frequency value upon afrequency shift of the intermediate frequency energy towards saidrejection frequency value.

5. In combination with a source of signal waves and a load circuit, awave transmission network coupling the source and load circuit, saidnetwork comprising a plurality of resonant circuits arranged in cascade,and said cascaded circuits including at least two in number, at leasttwo reactances coupling said resonant circuits, said reactances being ofopposite sign and in phase opposition at a frequency spaced from thetransmitted wave frequency by a predetermined frequency value thereby tosecure substantially complete rejection of wave energy at said spacedfrequency, and means, responsive to a frequency shift of the wave energyfrom the operating wave frequency, for automatically decreasing saidpredetermined frequency value.

G. In a superheterodyne receiver of the type including at least a firstdetector, an intermediate frequency amplifier and a second detector, anintermediate frequency transmission network between the first detectorand intermediate frequency amplifier, said network comprising at leasttwo resonant circuits each tuned to an operating intermediate frequency,means for coupling said tuned circuits in such a manner thatsubstantially innite rejection is produced of one side of theintermediate frequency by a distance of approximately 10 kilocycles, asecond intermediate frequency transmission network between theamplifier' and second detector, said second network comprising at leasttwo resonant circuits each tuned to said operating frequency, and meansfor coupling the latter in such a manner that substantially inniterejection is produced on the other side of the operating frequency bysaid frequency distance, and said second detector being constructed andarranged to produce a direct current voltage whose magnitude andpolarity depends upon the amount and sense of the frequency departure ofthe intermediate frequency energy from said operating frequency, andmeans for utilizing said direct current voltage for decreasing at willthe frequency distance between either one of said rejection frequenciesand said operating frequency.

EARL I. ANDERSON. GARRARD MOUNTJOY.

